Catalytic method and apparatus

ABSTRACT

A catalytic reaction system for rapid light-off of hydrocarbon conversion reactions, said reaction system comprising at least one catalyst element of sufficiently low thermal mass to be electrically heatable at the rate of at least 100 degrees Kelvin per second with less than about 50 watts of power per square centimeter of catalyst area perpendicular to the direction of flow.

CROSS-REFERENCE TO RELATED CASE

This application is a continuation-in-part of my copending U.S. patentapplication Ser. No. 273,343 filed Nov. 18, 1988, now issued as U.S.Pat. No. 5,051,241.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to improved catalytic reaction systems and tomethods for catalytic reaction of carbon containing compounds. In onespecific aspect the present invention relates to quick light-off, fastthermal response catalysts for use in catalytic exhaust gas reactors andin catalytic fuel combustion systems.

In one still more specific aspect, this invention relates to low thermalmass electrically conductive catalysts suitable for rapid electricalheating to operating temperature.

2. Brief Description of the Related Art

Automotive emissions are still a major environmental problem in spite ofthe major advances brought about by the use of catalytic converters. Onefactor limiting the performance of catalytic converters is thatpollution is not controlled during the thirty or so seconds required tobring the converter catalyst to its operating temperature. In presentconverters, warm-up is dependent on heating of the catalyst by hotengine exhaust gases. Although electrical heating can be utilized topreheat the catalyst prior to engine operation, the power and the timedelay required with present catalyst structures, ceramic or metal, havebeen deemed unacceptable.

The need to reduce catalyst warm-up time of the conventional ceramicmonolith automotive catalysts to reduce emissions during the warm-upperiod has led to increased interest in metal monolith catalysts.However, merely substituting metal for ceramic in a conventionalmonolith structure yields catalysts which still have much too high athermal mass. Although metal monoliths are electrically conductive andcould therefore be electrically preheated, fast enough heat up timeshave not yet been demonstrated as feasible with conventional monoliths.Even if sufficient electrical power were made available, thermal shockdamage would likely be a problem if a conventional metal monolith wereheated as rapidly as needed for elimination of start-up emissions, evenwith use of a conventional miniature catalyst suitable for only partialconversion of start-up emissions. Thus there is a critical need for acatalyst system which can control hydrocarbon emissions during initialengine operation without the need for heating prior to engine cranking.

For catalytic combustors the problem is not just emissions but theability to function in certain applications. For example, an automotivecatalytic combustor gas turbine must start in roughly the same timeframe as present automotive engines.

The present invention provides catalysts and systems which make possiblemuch more rapid warm-up of converter catalysts without electricalheating and near instantaneous electrical heating of catalysts incombustors, catalytic converters and other chemical reaction systems.

SUMMARY OF THE INVENTION Definitions of Terms

In the present invention the terms "monolith" and "monolith catalyst"refer not only to conventional monolithic structures and catalysts suchas employed in conventional catalytic converters but also to anyequivalent unitary structure such as an assembly or roll of interlockingsheets or the like.

The terms "low thermal mass catalyst" and "microtherm" used herein referto a thermally shock resistant catalyst element or structure which canbe heated from 300 degrees Kelvin to 800 degrees Kelvin at a rate of atleast one hundred degrees Kelvin per second with a power input of lessthan fifty watts per square centimeter of open area in the planeperpendicular to the direction of flow.

The terms "microlith" and "microlith catalyst" refer to high open areamonolith catalyst elements with flow paths so short that reaction rateper unit length per channel is at least fifty percent higher than forthe same diameter channel with a fully developed boundary layer inlaminar flow, i.e. a flow path of less than about two mm in length,preferably less than one mm or even less than 0.5 mm and having flowchannels with a ratio of channel flow length to channel diameter lessthan about two to one, but preferably less than one to one and morepreferably less than about 0.5 to one. Channel diameter is defined asthe diameter of the largest circle which will fit within the given flowchannel and is preferably less than one mm or more preferably less than0.5 mm. Microlith catalysts may be in the form of woven wire screens,pressed metal or ceramic wire screens or even pressed thin ceramicplates and have as many as 100 to 1000 or more flow channels per squarecentimeter. Flow channels may be of any desired shape. For wire screens,flow channel length is the wire diameter and thus advantageously may beshorter than 0.3 mm or even shorter than 0.1 mm.

The terms "carbonaceous compound" and "hydrocarbon" as used in thepresent invention refer to organic compounds and to gas streamscontaining fuel values in the form of compounds such as carbon monoxide,organic compounds or partial oxidation products of carbon containingcompounds as well as conventional hydrocarbon fuels.

The Invention

It has now been found that use of the microtherm catalysts of thepresent invention makes possible rugged catalyst systems which can bebrought to operating temperature in less than five seconds or even inless than one to two seconds with relatively little power. To achievesuch results requires not only that the thermal mass, i.e.; the weight,of the catalytic element be low but also that the geometry be such as tominimize the thermal stresses which result from such extremely rapidheating, i.e.; the flow channel length must be no longer than aboutthree channel diameters to minimize stresses resulting from axialtemperature gradients in use.

It has been found that use of the microlith microtherm catalysts of thepresent invention makes possible as much as a ten fold or more reductionin catalyst mass as compared to that required to achieve the sameconversion in mass transfer limited reactions of hydrocarbons usingconventional monoliths. It has been found that the specific masstransfer rate increases as the ratio of channel length to channeldiameter of a monolith catalyst is reduced below about five to one ormore preferably below about two to one and especially below about one toone. Mass transfer of reactants to the surface becomes sensitive to theinlet flow rate rather than being significantly limited by the diffusionrate through a thick laminar flow boundary layer as in conventionalmonolith catalysts, whether ceramic or metal. In such conventionalautomotive monolith catalysts, the amount of pollutants oxidized isessentially independent of exhaust gas flow rate and thus percentconversion decreases with increase in flow rate. In contrast, in themicrolith catalysts of the present invention, the amount of reactantsoxidized typically increases with increase in flow rate. Thus if theinlet flow velocity is high enough, the reaction rate can even approachthe intrinsic kinetic reaction rate at the given catalyst temperaturewithout imposing an intolerable pressure drop. This means that it ispractical to design microlith fume abatement reactors for much higherconversion levels than is feasible with conventional catalyticconverters. Conversion levels of 99.9% or even higher are achievable ina microlith automotive converter smaller in size than a lower conversionlevel conventional catalytic converter. Conversion levels high enoughfor abatement of toxic fumes are achievable in compact reactors.

With the short flow paths of catalysts of the present invention,pressure drop is low permitting the use of much smaller channeldiameters for a given pressure drop, further reducing catalyst massrequired. It has also been found that channel walls as thin as 0.1 mm oreven less than 0.03 mm are practical with small channel diameters thuspermitting high open areas even with such small channel diameters. Thus,as many as several thousand flow channels per square centimeter or evenmore are feasible without reducing open area in the direction of flowbelow sixty percent. Open areas greater than 65, 70 or even 80 percentare feasible even with high channel density microliths.

Inasmuch as heat transfer and mass transfer are functionally related, anincrease in mass transfer results in a corresponding increase in heattransfer. Thus, not only is catalyst mass reduced by use of themicrolith catalysts of this invention, but the rate at which anautomotive exhaust catalyst is heated by the hot engine exhaust iscorrespondingly enhanced.

Thus microlith catalysts are especially suitable microtherm catalysts inapplications requiring the maximum mass transfer rates as in fumeabatement applications whereas longer channel length catalysts arerequired, for example, where structural rigidity is needed. To achieve agiven thermal mass per unit length requires lower channel wallthicknesses for smaller channel diameters than for larger ones. Althoughmicroliths are often preferred for the electrically heated microthermelements in automotive or fume abatement applications, large combustorapplications, for example, often require larger cell size microthermelements with flow channel lengths as long as seven to ten millimetersto provide adequate structural rigidity.

If placed sufficiently close to an engine exhaust manifold, the amountof electrical power required to bring a microtherm catalyst system to aneffective operating temperature within seconds of cranking issubstantially reduced as a result of earlier contact with combustibles.Effective operating temperature for automotive exhaust precious metalcatalysts such as platinum and the like are as low as 650 or even as lowas 550 degrees Kelvin. However, an important feature of microthermcatalysts is that high enough operating temperatures are achievableprior to or during engine cranking to permit effective use of evendeactivated catalysts requiring a catalyst light-off temperature as highas 800 degrees Kelvin or more. Even temperatures high enough for use ofbase metal catalysts are readily achievable. It has been found that ametal microtherm element composed of a high temperature alloy containingcatalytic elements such as chromium, cobalt, copper, manganese, nickelor a rare earth metal can be sufficiently catalytically active for someuses if heated to a temperature of about 800 degrees Kelvin, atemperature readily achieved in less than one second with electricalheating. Many such alloys are commercially available and include Haynesalloy 25, Inconel 600, and even certain stainless steels. With metalmicrotherms, alloy selection is often determined primarily by oxidationresistance at the maximum operating temperature required by the givenapplication. Such base metal catalysts are especially useful incombustor applications. Fume abatement applications may use either knownbase metal or precious metal catalysts depending on the compound to bedestroyed and the minimum operating temperature required.

The mass of microtherm catalyst elements can be so low that it isfeasible to electrically preheat the catalyst to an effective operatingtemperature in as little as about 0.50 seconds if a thin channel wallelectrically conductive catalyst, e.g.; metal microtherm elements areused. In catalytic combustor applications the low thermal mass ofcatalyst elements of the present invention makes it possible to bring acombustor catalyst up to a light-off temperature as high as 1000 or even1500 degrees Kelvin in less than about five seconds by electricalheating and even in less than about one or two seconds using the powerfrom a conventional automotive battery. Such rapid heating is allowablefor microtherm catalysts because sufficiently short flow paths permitrapid heating without the consequent thermal expansion resulting indestructive stress levels, as for example as a result of leading edgecooling by the inlet gas flow.

In catalytic combustor applications, where unvaporized fuel droplets maybe present, flow channel diameter is often large enough to minimizeimpingement of the largest expected fuel droplet. Therefore in catalyticcombustor applications flow channels may be as large as three or fourmillimeters in diameter whereas in automotive catalytic converterapplications, much smaller flow channels are preferred. If desired, one,two or three microtherm catalyst elements may be placed in front of aconventional monolith catalyst element to serve as a light-off reactorfor a downstream conventional monolith catalyst.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIG. 1 shows a face view of an electrically conductive microthermcatalyst with electrical leads attached. FIG. 2 is a block diagramshowing a system for hydrocarbon conversions incorporating themicrotherm catalyst of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

The present invention is further described in connection with thedrawing. As shown in the FIG. 1, in one preferred embodiment amicrotherm catalyst element 10 comprises a plurality of square flowchannels 11 with electrical leads 15 connected to bus bars 16. Bus bars16 are welded at a forty five degree angle to metallic flow channelwalls 12 to ensure even heating of catalyst 10. Advantageously, catalystelement 10 is in the form of a catalytic metal honeycomb like structurewith wall thickness thin enough to provide an open area of at leastabout 60% and more preferably at least about 65 to 70%. Using the powerof a standard automotive battery the catalyst may be brought to aneffective operating temperature in less than one second, often insignificantly less than 0.50 seconds. Thus in automotive exhaust gasservice, electrical power need not be applied till just after start ofcranking thus limiting maximum drain on the battery. Advantageously,electrical power is applied prior to termination of engine cranking.Typically, an automotive microlith catalyst element is heated to aneffective operating temperature within one to two seconds of start ofcranking. This rapid heating is important in that no delay in enginestarting is required to achieve emissions control. Typical reactors mayhave from one to ten or more such microliths.

The microtherm catalysts of the present invention are readily made usingknown catalytic agents. The following examples describe means of makingmicrotherm catalysts but are not to be construed as limiting. Amicrotherm catalyst as per the drawing may be made by vacuum sputteringa catalyst such as platinum onto a stainless steel microtherm honeycombwhich has been cleaned by heating in air to 750 degrees Kelvin.Typically the platinum coating may be from 5 to 100 angstroms inthickness but may be thicker for greater catalyst life. Advantageously,a similarly thin layer of ceria or alumina may be deposited on thesubstrate prior to deposition of the platinum. Catalysts containingpalladium, iridium, rhodium or other metals can be similarly prepared.In many applications, especially for higher catalyst operatingtemperatures, a microtherm element formed from stainless steel or otheralloy is a sufficiently active catalyst without additional coating.Although metal microtherms are preferred, microtherms of electricallyconductive ceramics can be made by slicing of ceramic honeycombextrudates prior to firing. Such ceramic honeycomb extrudatesadvantageously may contain an organic binder to facilitate production ofthin slices. As necessary for sufficient low temperature catalyticactivity, ceramic and metal microtherm elements may be made catalyticusing various techniques well known in the art.

EXAMPLE 1

A three element catalytic microlith automotive exhaust reactor havingabout 2500 flow channels per square centimeter is constructed using afive centimeter wide strip of 70% open area screening of platinum coatedstainless steel wires having a diameter of 0.03 mm spaced 0.20 mm apartand installed in the exhaust pipe of a four cylinder automotive engineas a means of directing a hydrocarbon flow to the catalyst see FIG. 2.During engine cranking electrical power from the battery is appliedheating the microlith catalyst elements to a temperature of 700 degreesKelvin within one second whereby hydrocarbon emissions are controlledduring initial operation of the engine.

EXAMPLE II

An electrically heated two element microtherm catalytic combustor isconstructed using microtherm catalytic elements four millimeters inlength with hexagonal channels 1.5 millimeters in diameter. Themicrotherm elements are electrically heated to 900 degrees Kelvin withinthree seconds and an intimate mixture of fuel and air is formed byspraying jet fuel into the air passing into the reactor. Plug flow gasphase combustion of the fuel is achieved. Advantageously, an electronictemperature controller is used to limit the catalyst temperature duringelectrical heating and to assure maintaining an effective temperature inthe event of a fuel interruption of other flow upset. A controller withanticipatory capabilities is preferred.

EXAMPLE III

A fume abatement reactor six centimeters in length is constructed using300 microlith elements of screening with about thirty 0.050 mm wires ofplatinum coated nichrome per centimeter (nominally 900 flow channels persquare centimeter). Fumes containing 50 ppm by volume of benzene in airare preheated to 700 degrees Kelvin and passed through the microlithreactor. Better than 99.9 percent conversion of the benzene is achieved.

What is claimed is:
 1. A catalytic reaction system for rapid light-offof hydrocarbon conversion reactions, which comprises;a microlithcatalyst element having a plurality of open flow channels, each withachannel flow diameter of less than about 1.5 mm; and a channel flowlength no longer than about 3 channel diameters; said element havingsufficiently low thermal mass to be electrically heatable at the rate ofat least 100 degrees Kelvin per second as measured within a temperaturerange of 300 to 1000 degrees Kelvin, using a power of less than about 50watts per square centimeter of catalyst element area perpendicular tothe direction of hydrocarbon flow; and means to electrically heat saidcatalyst element from ambient temperature to a temperature within therange of from 550° K. to 1500° K. within 5 seconds time of applying themeans, connected electrically to the microlith catalyst element.
 2. Thesystem of claim 1 wherein said power is less than about 25 watts persquare centimeter of said area perpendicular to the direction of flow.3. The system of claim 1 wherein the time to heat to said temperature isless than about three seconds.
 4. The system of claim 3 wherein saidtemperature is greater than 800 degrees.
 5. The system of claim 1 inwhich the open channels of said catalyst element is greater than about65 percent of the element area transverse to the direction ofhydrocarbon flow.
 6. The system of claim 1 wherein said catalyst elementcomprises an electrically conductive ceramic.
 7. The system of claim 1wherein said catalyst comprises a metal catalyst.
 8. The system of claim1 in which the number density of said flow channels is greater than 100channels per square centimeter.
 9. The system of claim 8 in which thenumber density of said flow channels is greater than 1000 channels persquare centimeter.
 10. The system of claim 1 wherein the mass of saidcatalyst element is sufficiently low such that said catalyst element canbe electrically heated to a temperature of at least 1000 degrees Kelvinwithin about one second using an automotive battery.
 11. The system ofclaim 10 wherein the mass of said electrically conductive catalystelement is sufficiently low such that said catalyst element can beheated to a temperature of at least 1000 degrees Kelvin within about0.50 seconds.
 12. A catalytic reaction system of claim 1 wherein theflow channel diameter is less than 1 mm.
 13. A catalytic reaction systemof claim 1 wherein the flow channel diameter is less than 0.5 mm.
 14. Acatalytic reaction system of claim 1 wherein the flow channel length isshorter than 0.3 mm.
 15. A catalytic reaction system of claim 1 whereinthe flow channel length is shorter than 0.1 mm.
 16. The method ofcontrolling combustion exhaust emissions from internal combustionengines during initial engine operation, which comprises;a. providing amicrolith catalyst element having a plurality of open flow channels;each witha channel flow diameter of less than about 1.5 mm; and achannel flow length no longer than about 3 channel flow diameters; saidelement having a sufficiently low thermal mass to be electricallyheatable at the rate of at least 100 degrees Kelvin per second asmeasured within a temperature range of 300 to 800 degrees Kelvin, usingless than about 50 watts of power per square centimeter of catalystelement area perpendicular to the direction of channel flow; b. startingthe engine, c. electrically heating the catalyst element to an effectivecatalyst operating temperature within three seconds of starting theengine; and d. passing the exhaust emissions through said catalystelement flow channels.
 17. The method of claim 16 wherein said catalystelement reaches catalyst operating temperature within one second ofstarting the engine.
 18. The method of claim 16 wherein the channel flowdiameter is less than 1 mm.
 19. The method of claim 16 wherein thechannel flow diameter is less than 0.5 mm.
 20. The method of claim 16wherein the flow channel length is shorter than 0.3 mm.
 21. The methodof claim 16 wherein the flow channel length is shorter than 0.1 mm.